1. Field of the Invention
This invention relates generally to position location and providing services based on position location. More specifically, it relates to providing services in cases where the position location is based on DTV signals.
2. Description of the Related Art
There have long been methods of two-dimensional latitude/longitude position location systems using radio signals. In wide usage have been terrestrial systems such as Loran C and Omega, and a satellite-based system known as Transit. Another satellite-based system enjoying increased popularity is the Global Positioning System (GPS).
Initially devised in 1974, GPS is widely used for position location, navigation, survey, and time transfer. The GPS system is based on a constellation of 24 on-orbit satellites in sub-synchronous 12 hour orbits. Each satellite carries a precision clock and transmits a pseudo-noise signal, which can be precisely tracked to determine pseudo-range. By tracking 4 or more satellites, one can determine precise position in three dimensions in real time, world-wide. More details are provided in B. W. Parkinson and J. J. Spilker, Jr., Global Positioning System-Theory and Applications, Volumes I and II, AIAA, Washington, D.C., 1996.
GPS has revolutionized the technology of navigation and position location. However in some situations, GPS is less effective. Because the GPS signals are transmitted at relatively low power levels (less than 100 watts) and over great distances, the received signal strength is relatively weak (on the order of −160 dBw as received by an omni-directional antenna). Thus the signal is marginally useful or not useful at all in the presence of blockage or inside a building.
There has even been a proposed system using conventional analog National Television System Committee (NTSC) television signals to determine position. This proposal is found in a U.S. patent entitled “Location Determination System And Method Using Television Broadcast Signals,” U.S. Pat. No. 5,510,801, issued Apr. 23, 1996. However, the present analog TV signal contains horizontal and vertical synchronization pulses intended for relatively crude synchronization of the TV set sweep circuitry. Further, in 2006 the Federal Communication Commission (FCC) will consider turning off NTSC transmitters and reassigning that valuable spectrum so that it can be auctioned for other purposes deemed more valuable.
Knowledge of an object or individual's position is useful or even required in providing certain services. For example, emergency 911 services require knowledge of the caller's location. Similarly, navigation services typically require knowledge of the user's position. Other services, such as traffic reports and concierge services, can be provided without knowledge of an individual's location but can be greatly enhanced if the user's location is known.
However, the current position location systems have significant drawbacks that prevent or inhibit their use for many of these services. For example, the proposed NTSC system is not suitable for services that require high precision. In addition, since the NTSC transmitters may be turned off in 2006, services that require large initial investments are also less attractive since the time to recover the investment may be limited. GPS systems generally have higher precision than the NTSC system. However, the receivers are relatively complex and expensive, thus making the GPS approach inappropriate for services that require simple and/or low-cost receivers. In addition, the GPS signal is not particularly robust. GPS' low signal power makes it marginally useful in the presence of blockage or inside buildings. Its low bandwidth signal also makes it susceptible to multipath effects. The GPS satellites also travel at high speeds and significant Doppler aiding may be required to determine their exact location. At times, GPS geometries may be insufficient, leading to high Geometric Dilution of Precision (GDOP) and making it difficult to provide highly accurate position fixes. These factors make GPS unsuitable for many types of services.